U.S. patent number 5,434,905 [Application Number 08/159,661] was granted by the patent office on 1995-07-18 for digital cordless telephone set operated under burst synchronization.
This patent grant is currently assigned to Uniden Corporation. Invention is credited to Masayasu Fujino, Kazuo Maeda, Kiyoshi Tanaka, Jun Yang.
United States Patent |
5,434,905 |
Maeda , et al. |
July 18, 1995 |
Digital cordless telephone set operated under burst
synchronization
Abstract
A cordless telephone set comprises a base unit and a handset
unit connected by a radio frequency (RF) link which is established
by digital burst data transmission and is synchronized by a unique
word which is included in the digital burst data. The radio
transmission signal is not continuous and it requires the
synchronization. The unique word is detected by the unique word
detecting means and is used to establish the burst data
synchronization. The unite word consists of frequency channel
information and fixed unite data. By using this information, the
base unit and the handset unit are prevented from trying to
establish a radio link at incorrect frequency when a spurious
signal, not intended for the base unit and/or handset unit is
erroneously received.
Inventors: |
Maeda; Kazuo (Ichikawa,
JP), Fujino; Masayasu (Ichikawa, JP), Yang;
Jun (Ichikawa, JP), Tanaka; Kiyoshi (Ichikawa,
JP) |
Assignee: |
Uniden Corporation
(JP)
|
Family
ID: |
22573447 |
Appl.
No.: |
08/159,661 |
Filed: |
November 30, 1993 |
Current U.S.
Class: |
455/464; 455/411;
370/350 |
Current CPC
Class: |
H04B
7/2681 (20130101); H04W 56/0085 (20130101); H04M
1/72505 (20130101); H04L 7/046 (20130101); H04W
76/10 (20180201); H04W 84/16 (20130101) |
Current International
Class: |
H04B
7/26 (20060101); H04M 1/725 (20060101); H04Q
7/38 (20060101); H04M 1/72 (20060101); H04L
7/04 (20060101); H04Q 007/22 (); H04M 011/00 () |
Field of
Search: |
;379/59,61,63,58
;455/83,33.1,54.1 ;375/8,365 ;370/95.1,95.3,105.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
55-153436 |
|
Nov 1980 |
|
JP |
|
58-6634 |
|
Jan 1983 |
|
JP |
|
3-69218 |
|
Mar 1991 |
|
JP |
|
3-270333 |
|
Dec 1991 |
|
JP |
|
5-48513 |
|
Feb 1993 |
|
JP |
|
Primary Examiner: Kurtz; Curtis
Assistant Examiner: Trost; William G.
Attorney, Agent or Firm: Ratner & Prestia
Claims
The invention claimed is:
1. A digital cordless telephone set having a base unit which
includes a terminal connected to a telephone network line and a
battery-powered handset unit, each of the base unit and the handset
unit comprising,
frequency channel selecting means for selecting a frequency channel
for communication,
frequency channel information generating means for generating
digitally encoded frequency channel information indicating a
frequency to be used as the frequency channel selected by the
frequency channel selecting means,
frequency channel information transmitting means for transmitting
the digitally encoded frequency channel information generated by
the frequency channel generating means,
frequency channel information receiving means located in the base
unit and in the handset unit for receiving the digitally encoded
frequency channel information transmitted from the frequency
channel information transmitting means of the handset and base
unit, respectively,
reference frequency channel information setting means for setting
reference frequency channel information based on the frequency
channel selected by the frequency channel selecting means,
frequency channel information detecting means for detecting when
the reference frequency channel information matches the digitally
encoded frequency channel information received by the frequency
channel information receiving means, and
burst frame synchronization establishing means for establishing
burst frame synchronization using a unique word in a burst data
stream,
wherein the digitally encoded frequency channel information is
included in the unique words transmitted by the base unit and the
handset unit respectively: and the burst frame synchronization is
established when the digitally encoded frequency channel
information detected by the frequency channel information detecting
means matches the reference frequency channel information.
2. A digital cordless telephone set according to claim 1 , wherein
the unique word includes bits corresponding to the digitally
encoded frequency channel information and fixed pattern bits.
3. A digital cordless telephone set having a base unit which
includes a terminal connected to a telephone network line and a
battery-powered handset unit, each of the base unit and the handset
unit comprising,
frequency channel selecting means for selecting a frequency channel
for communication,
unique word generating means for generating a unique word including
fixed pattern bits and frequency channel information bits
determined by the frequency channel selected by the frequency
channel selecting means,
burst data stream transmitting means for transmitting a burst data
stream including the unique word,
burst data stream receiving means located in the handset unit and
in the base unit for receiving the burst data stream transmitted
from the burst data stream transmitting means of the base unit and
handset unit, respectively,
reference unique word setting means for setting a reference unique
word based on the frequency channel selected by the frequency
selecting means, wherein the digitally encoded frequency channel
information is included in the unique words transmitted by the base
unit and the handset unit respectively:
unique word detecting means for detecting the unique word from the
received burst data stream by comparing the reference unique word
set by the reference unique word setting means with the received
burst data stream, and
burst frame synchronization means for generating timing signals to
synchronize the received burst data stream and for extracting
information data including control data and voice data from
portions of the burst data stream following the unique word,
wherein the burst frame synchronization is established when the
digitally encoded frequency channel information detected by the
unique word detecting means matches the reference unique word
information.
4. A digital cordless telephone set according to claim 3, wherein
the unique word consists of frequency channel information bits
determined by the frequency channel information bits and the fixed
pattern bits.
5. A cordless telephone set according to claim 3 or 4 wherein,
the frequency channel information bits are encoded using an
error-correcting code, and
the unique word detecting means comprises an error correction
circuit for correcting errors in the received burst data stream
before the burst data is compared with the reference unique
word.
6. A digital cordless telephone set according to claim 3 further
comprising,
spread code generating means for generating a spread code sequence
having a plurality of positions,
data spreading means for assigning, to each position in the spread
code sequence, a respective baseband data bit, and
data de-spreading means for changing the received burst data stream
into the baseband data by comparing the burst data stream with the
spread code sequence.
7. A digital cordless telephone set according to claim 6 wherein
the spread code sequence is encoded based on the frequency channel
provided by the frequency channel selecting means.
8. A digital cordless telephone set comprising:
a hand set unit including:
frequency channel selecting means for selecting a frequency channel
for communication,
frequency channel information generating means for generating
digitally encoded frequency channel information indicating a
frequency to be used as the frequency channel selected by the
frequency channel selecting means, and
frequency channel information transmitting means for transmitting
the digitally encoded frequency channel information generated by
the frequency channel generating means; and
a base unit including:
frequency channel information means for receiving the digitally
encoded frequency channel information transmitted from the hand set
unit,
reference frequency channel information setting means for setting
reference frequency channel information based on the received
digitally encoded frequency channel information, and
means for establishing burst frame synchronization between the base
unit and the handset unit if the received digitally encoded
frequency channel information corresponds to the reference
frequency channel information.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a cordless telephone set, and
particularly relates to a digital cordless telephone set which
requires a synchronization to digital burst transmission data such
as TDM (Time Division Multiplex) data.
Description of the Prior Art
A digital cordless telephone set comprises a base unit which may be
connected to a telephone network and a battery-powered handset unit
which is connected by a radio frequency (RF) link to the base unit.
The RF link is established by sending and receiving a digitally
modulated signal between the base unit and the handset unit.
Most digital cordless phones use multiple channels by various
multiplexing methods. TDM is one of these methods in which a short
transmission data burst is assigned to one of several time frames.
TDD (Time Division Duplex) which is also known as Ping-Pong
transmission, is an example of TDM which has an advantage of using
only one frequency channel for transmitting and receiving. FIG. 1
shows an example of a burst assignment in Ping-Pong transmission
between the base unit and the handset unit. As shown in this
Figure, the channel is alternately used to transmit and receive
respective data signals.
FIG. 2 shows an example of the burst transmission data stream
consisting of preamble bits (P), unique word (UW), status bits (S),
information data (I), check bits (CHK) and guard bits (G). The
preamble bits are used for bit clock recovery, the unique word is
used for frame synchronization and the status bits are used for
associated control data communication. The information data
includes digital voice data and control data, and the check bits
are usually used to detect bit errors in the burst frame. The guard
bits are empty bits which are used to prevent overlap between the
transmitting frame timing and the receiving frame timing.
For an exemplary prior-art cordless phone, the frame
synchronization should be established before communication is
started. The frame synchronization is established using the unique
word.
The receiver first recovers the bit clock from the preamble bits
(bit length Lp), the pattern of the preamble bits is, for example,
the repetition of "1" and "0" as used in FSK and PSK schemes. The
received burst data is sampled in the clock timing determined by
the bit clock to output the data.
Next, the receiver locates the unique word (bit length Lu) in the
received burst data. Once the unique word is detected, the receiver
recognizes that the data after the unique word are the status bits
and the information data, and the starting point of the received
burst data. That is to say the timing for receiving the burst data
is the instant which is (Lp+Lu) bits before the first one of the
status bits. The receiver also recognizes that the time for
transmitting the burst data is one burst frame length after the
beginning of receive timing. Following the scheme, the receiver
knows the correct timing to send transmission burst data. In this
manner, the frame synchronization is established.
In a normal operation of the digital cordless telephone set, when
the transmitter, for example, the base unit, sends burst data at a
frequency of F1, the handset unit is going to receive the burst
data while scanning frequency channels in order to communicate with
the base unit at the frequency of F1.
However, cordless phones are used at various location.
Occasionally, the handset unit is located very near to the base
unit. In this case, the transmission data may be received at an
incorrect frequency because the strength of the signal received at
the base station is much greater than during normal operation of
the cordless phone. This incorrect reception is one type of
spurious reception. Another type of spurious reception is an image
reception which occurs in a heterodyne receiving system. Spurious
reception may cause problems because, if the cordless phone
receives the signal at the incorrect frequency, it will send the
transmission data at the incorrect frequency and will never
establish the radio link.
For example, if the base unit sends transmission data using TDD at
a frequency of F1, and the handset unit receives this data at a
frequency of F2 due to spurious reception, the handset will send
reply data at the frequency of F2. The base unit is waiting for the
reply at the frequency of F1, however, and will newer receive the
reply.
Furthermore, such misconnection at a different frequency may
interfere with other communications. In the example presented
above, this interference would be with another cordless telephone
operating at frequency F2.
SUMMARY OF THE INVENTION
Accordingly it is an object of the present invention to prevent a
digital cordless telephone from failing to establish a connection
due to spurious reception of signals at an improper frequency.
It is an another object of the present invention to provide a
digital cordless telephone set without spurious reception at an
improper frequency.
Each controller of the base unit and the handset unit sets a unique
word in its transmitted data. This unique word includes the channel
frequency information which shows a frequency to be used to
communicate between the base unit and the handset unit, the
remaining portion of the unique word is a fixed pattern. The unique
word is programmable by the controller.
The base unit and the handset unit may recognize, based on the
unique word, whether or not the received signal is a desired signal
at a correct frequency. As a result, the digital cordless telephone
may prevent spurious receptions.
The digital cordless telephone set according to the present
invention comprises frequency, channel selecting means for
selecting a frequency channel for communication,
unique word generating means for generating a unique word including
fixed pattern bits and frequency channel information bits
determined by the frequency channel that is selected by the
frequency channel selecting means,
burst data stream transmitting means for transmitting a burst data
stream including the unique word,
burst data stream receiving means for receiving the burst data
stream transmitted from the burst data stream transmitting
means,
reference unique word setting means for setting a reference unique
word based on the frequency channel selected by the frequency
selecting means,
unique word detecting means for detecting the unique word from the
received burst data stream by comparing the reference unique word
set by the reference unique word setting means with the received,
burst data stream, and
burst frame synchronization means for generating timing signals to
synchronize the received burst data stream and for extracting
information data including control data and voice data from
portions of the burst data stream following the unique word.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a timing diagram which shows a burst assignment in
Ping-Pong transmission between the base unit and the handset
unit.
FIG. 2 is a bit allocation diagram which shows an example of the
burst transmission data stream.
FIG. 3 is a block diagram of the base unit of the cordless
telephone set.
FIG. 4 is a block diagram of the handset unit of the cordless
telephone set.
FIG. 5 is a bit allocation diagram which shows the format of the
unique word.
FIG. 6 is a block diagram of the framer circuit shown in FIGS. 3
and 4.
FIG. 7 is a block diagram of an exemplary unique word detector.
FIG. 8 is a block diagram of an alternative embodiment of the base
or handset unit using a spread spectrum scheme.
DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
FIGS. 3 and 4 show respectively the base unit and the handset unit
of the digital cordless telephone set using TDD. The base unit and
the handset unit have substantially the same construction, so the
same element is denoted by the same reference number in both
figures, but the reference numerals in FIG. 4 are primed.
In the base unit shown in FIG. 3, the analog voice signal from a
telephone network line 1 provided through an interface 2, is
changed to a digital voice signal by the voice CODEC 3. Then, the
preamble data, unique word, status data and check bits are added
serially to the digital voice signal by the framer circuit 4.
Exemplary circuitry suitable for use as the frame circuit 4 is
described below with reference to FIG. 6. This circuit may be, for
example, a multiplexor/demultiplexor. The signal produced by the
framer circuit 4 is baseband transmission data. The unique word,
which is added to this data by the framer circuit, consists of the
frequency channel information bits and the fixed pattern bits as
shown in FIG. 5.
The frequency channel information bits indicate the frequency used
to communicate between the handset unit and the base unit, and the
pattern of these bits is different for each frequency channel. On
the other hand, the pattern of the fixed pattern bits is the same
for all frequency channels. The frequency channel information bits
in each frequency channel are determined by the frequency channel
information stored for each channel in the memory 17.
When the base unit shown in FIG. 3 is going to communicate with the
handset unit, the controller 16 reads, from the memory 17, the
frequency channel information which corresponds to the frequency
channel to be used in communication. The controller 16 sends that
frequency channel information to the framer circuit 4.
The framer circuit 4 sets the unique word based on the frequency
channel information sent from the controller 16. The framer circuit
4 may speed up the bit rate, for example the input data of 100
kbit/sec is changed to 300 kbit/sec baseband transmission data. The
baseband transmission data is an intermittent signal which occupies
one-half of the burst frame cycle, the other-half of the burst
frame cycle is used for reception. The baseband transmission data
is modulated by the modulator 5, heterodyned up to the RF frequency
with the local oscillator signal from the local oscillator 6 by the
mixer 7, and then amplified by the radio frequency (RF) power
amplifier 8. Finally, the RF signal goes through the antenna switch
9 and is transmitted from the antenna 10. The antenna switch 9 is
interchangeably switched between transmitting and receiving modes
based on a TX/RX control signal from the framer circuit 4, the
TX/RX control signal is generated based on the frame
synchronization. The framer circuit 4 and the local oscillator 6
are controlled by the controller 16.
In the receive half cycle of the burst frame, the signal received
by the antenna 10 goes through the antenna switch 9 and is
amplified by the front-end amplifier 11. Then, it is mixed with the
local oscillator signal from the local oscillator 6 in the mixer 12
to produce an intermediate frequency (IF) signal. The IF signal is
filtered by the IF filter 13 and amplified by the IF amplifier 14.
The amplified IF signal is input to the demodulator 15. The
demodulator 15 demodulates the IF signal to recover the baseband
receive data and outputs this data to the framer circuit 4 and the
clock recovery circuit 18.
The framer circuit 4 detects the unique word from the baseband
receive data, and makes use of the frequency information it
contains to match the receiving frequency with the correct
frequency to be received in order to avoid the reception of an
incorrect frequency. The framer circuit may reduce the bit rate,
for example the 300 kbit/sec baseband received data is changed into
the 100 kbit/sec received voice data.
As described hereinbefore, the clock recovery circuit 18 recovers
the bit clock based on the preamble bits having the bit pattern of
"1" and "0" repetition for example, and outputs the bit clock to
the framer circuit 4. In the framer circuit 4, the received data is
processed based on the clock timing set by the bit clock.
If the framer circuit 4, based on information in the unique word,
recognizes that the correct frequency has been received, frame
synchronization is established, and the digital voice signal is
output from the framer circuit 4 in synchronization mode. The
digital voice signal is input to the CODEC 3, and the CODEC changes
the digital voice signal into the analog voice signal. The analog
voice signal is sent to the telephone network line 1 through the
interface 2.
The handset unit shown in FIG. 4 comprises a microphone 20 and a
speaker 21 in place of the interface 2 of FIG. 3, and further
comprises a keypad. The microphone 20 and the speaker 21 are
connected to CODEC 3' and the keypad 22 is connected to the
controller 16'. The operation of the handset unit is substantially
the same as that of the base unit shown in FIG. 3, so the
description thereof is omitted.
FIG. 6 shows the construction of the framer circuit 4 of the base
unit. The transmitting section of the framer circuit 4 comprises a
buffer 31, a preamble bit generator 32, a status bit generator 33,
a unique word generator 34, a check bit generator 35, a multiplexor
36 and a timing circuit 37. The receiving section of the framer
circuit 4 comprises a buffer 38, a demultiplexor 39 and a unique
word detector 40.
The construction of the framer circuit 4' of the handset unit is
the same as that of the framer circuit 4. Therefore, the operation
of only the framer circuit 4 is now described.
In the transmitting mode, the voice data from the CODEC 3 is first
stored in the buffer 31. The voice data from the buffer 31, the
preamble bits from the preamble bit generator 32, the status bits
from the status bits generator 33, the unique word from the unique
word generator 34 and the check bits from the check bit generator
35 are multiplexed in the multiplexor 36 to produce a frame of data
,to be transmitted. At this time, respective output timings of the
buffer 31 and the generators 32-35 are controlled by the timing
circuit 37. The timing circuit 37 is driven by a signaling output
pulse from the unique word detector 40, described below.
In the receiving mode, the baseband data from the demodulator 15 is
first stored in the buffer 38, and then is input to the
demultiplexor 39 and the unique word detector 30. The unique word
detector 40 detects the unique word included in the received
baseband data and compares the detected unique word with the
reference unique word 31 set by the controller 16 as described
above.
The unique word detector 40 outputs a signaling pulse to the
demultiplexor 39 and, to the timing circuit 37 when a matching
unique word is detected as the reference unique word in the
comparison.
Upon receipt of the signaling pulse from the unique word detector
40, the demultiplexor 39, in frame synchronization mode, extracts
the information data including the digital voice data and the
control data from the received baseband data and outputs it to the
CODEC 3. The demultiplexor 39 also extracts the status bits and
outputs them to the controller 16.
The operation of the cordless telephone system as shown in FIGS. 3,
4, and 6 is now described in detail from an off-hook condition to
the first data transfer.
When the off-hook key in the keypad 22 of the handset unit is
depressed, the controller 16' recognizes that the off-hook key has
been depressed and selects one of several frequency channel
information words stored in the memory 17'. The number of frequency
channel information words corresponds to the number of frequency
channels which may be used in communication, for example 16
channels.
The controller 16' provides the oscillation frequency data to the
local oscillator 6' based on the selected frequency channel
information in order to set the oscillation frequency of the local
oscillator 6'. The controller 16' also provides the information on
the selected frequency channel to the unique word generator 34. The
unique word generator 34 generates the unique word consisting of
the channel information bits and the fixed pattern bits, the
channel information bits being formed by the information on the
selected frequency channel.
The controller 16' instructs the status bit generator 33 to place
the information which shows the off-hook status of the handset unit
into the status bits. The framer circuit 4' generates the remaining
burst data including the unique word and the status bits other than
the preamble bits, the information data, the check bits and the
guard bits. The handset unit then sends this burst data to the base
unit.
In the base unit, a plurality of receiving frequency channels are
scanned in turn. This scanning is performed in such a manner that:
the controller 16 reads the first frequency channel information
word from the memory 17, and then sets the oscillation frequency of
the local oscillator 6 and the reference unique word 41 consisting
of the frequency channel information bits and the fixed pattern
bits based on the read frequency channel information word. The base
unit receives the signal at the first frequency channel for a
predetermined time period and determines if the received signal has
the same unique word as the reference unique word. If the channel
information read from the memory 17 of the base unit and the
channel information read from the memory read 17' of the handset
unit are the same, the unique word sent from the handset unit
matches the reference unique word set in the base unit. When the
received unique word does not match the reference unique word, the
base unit scans the next frequency channel.
When the base unit detects the same unique word as the reference
one during the scanning operation, the base unit stops the scanning
operation and establishes the frame synchronization before the
communication is started.
After such frame synchronization is established, the status bits
extracted by the demultiplexor 39 are applied to the controller 16,
the status bits include the information for the off-hook status of
the handset unit.
The controller 16 of the base unit knows that the hand set has been
placed in an off-hook state from the extracted status bits, and
causes the telephone line 1 to be placed in on off-hook state. As a
result, the base unit may begin sending data to the handset unit in
frame synchronization mode.
As described above, according to the preferred embodiments the
burst frame synchronization may be established by receiving the
same unique word as the reference one. After the burst frame
synchronization has been established, the base unit and the handset
unit may communicate with each other at the correct frequency.
In general, the unique word has relatively long bit length to
prevent from mistaking a noise signal for the unique word. It is
therefore very useful to adopt an idea of bit error detection for
allowing bit errors in the unique word. Using this technique, the
unique word detector 40 may recognize a unique word and output the
signaling pulse, even if there is a one bit error in the unique
word. If the unique word detector 40 comprises an error-correcting
system and the unique word is coded using an error-correcting code,
depending on the code that is used, two or more erroneous bits may
be allowed for the unique word.
When an error-correcting code (ECC) is used, the controller 16
(16') ECC encodes the channel information read from the memory 17
(17'). The unique word comprises ECC encoded channel information
bits plus the fixed pattern bits. Such unique word is detected by a
unique word detector having an error-correcting ability.
FIG. 7 shows one example of such unique word detector. This unique
word detector comprises a shift register 51, an error-correction
circuit 52 and a comparator 53.
The received baseband data is input in series into the shift
register 51. In the shift register 51, the data is shifted bit by
bit based on the bit clock, and the data in the shift register is
output in parallel to the error correction circuit 52. The circuit
52 corrects bit errors in the data and provides the error-corrected
data to the comparator 53.
At the same time, the ECC encoded reference unique word is
delivered to the comparator 53 from the controller. The data and
the reference unique word are compared in the comparator 53. When
the comparator detects the coincidence of the data and the
reference unique word, a detection pulse or a signaling pulse is
output.
An example of the error-detecting encoded unique word is now
described. It is assumed in the following that the number of
frequency channels is, at most, 16. In this case, the frequency
channel information bits of the unique word may consist of 4 bits
of binary coded data. The frequency channel information for channel
5 for example, is expressed as "0101". One method for
error-correcting encoding the frequency channel information is to
use Hamming code formed by generator polynomial G(X)=X.sup.3 +X+1.
For example, error-correcting encoding using Hamming code to the
frequency channel information "0101" for channel 5 results in 7
bits data of "0100111".
The fixed pattern of 15 bits data, "111100010011010" as an example
is added to the error-correcting encoded frequency channel
information bits of "0100111", resulting in 22 bits unique word of
"0100111111100010011010". It should be noted that the fixed pattern
may be considered to be essentially ECC encoded, because the number
possible fixed patterns may be limited to a small number, such as 4
or 5. Therefore, it is unnecessary to separately ECC encode the
fixed pattern.
The frame synchronization, described above, using a unique word
which includes frequency information can be applied to a digital
cordless telephone set using spread spectrum transmission
techniques.
FIG. 8 shows an example of the same portions of the base unit and
the handset unit of a digital cordless telephone set using direct
spread spectrum scheme. The construction in FIG. 8 is the same as
that shown in FIG. 3 except the portions of spectrum spread and
spectrum de-spread, i.e. the spread code generator 54, the
exclusive OR circuit 55 and the comparator 56. Other elements are
denoted by the same reference numerals in FIG. 3.
In the transmitter, the baseband data from the framer circuit 4 and
the spread code from the spread code generator 54 are exclusive
ORed in the exclusive OR circuit 55. The length of one spread code
and the time duration of one bit of the baseband data from the
framer circuit 4 are generally equal to each other, so that one
spread code sequence corresponds in time to one bit of the data
from the framer circuit 4. As a result of which, the exclusive ORed
data supplied to the modulator 5 has the higher data rate than that
of the data from the framer circuit 4 by the number of positions of
the spread code sequence. The modulated output data from the
modulator 5, therefore, is a signal which has been spread in
spectrum.
In the receiver, the demodulated output data from the demodulator
15 is compared with the spread code from the spread code generator
54 in the comparator 56. If the detected code is the same as or
similar to the spread code, the comparator 56 outputs the baseband
data to the framer circuit 4 and the clock recovery circuit 18.
In this embodiment, it is possible to establish the frame
synchronization using the unique word as described in FIGS. 3, 4
and 6.
If the spread code may be changed based on the frequency channel
information from the controller 16, it is also possible to
establish the frame synchronization using the spread code in place
of the unique word. In this case, the spread code is made
programmable by the controller 16 in order to change the spread
code based on the frequency channel information read from the
memory 17. For example, the controller 16 sets the local oscillator
6 to channel 1 and then assigns the PN-A sequence to the spread
code. In the same way channel 2 may be assigned the PN-B sequence,
and so on for the remaining channels.
In this system, when the spread code set in the transmitter and the
spread code set in the receiver are matched, the frame
synchronization is established and then the communication link is
established at a correct frequency. When both of the spread codes
do not match because of spurious reception at an incorrect
frequency, the receiver, based on the mismatch, may prevent the
spurious reception.
When frequency hopping is used as a spectrum spread scheme, the
controller may store the spread code, set the frequency of the
local oscillator based on the spread code, and write the frequency
channel information into the unique word. This system may be
implemented in the digital cordless telephone set shown in FIGS. 3
and 4.
* * * * *